1. Field of the Invention
The present invention relates to DC motor controllers, and, in particular, a motor control system that includes locked and lost rotor detection for a brushless and sensorless DC motor.
2. Description of the Related Art
For certain electric motor applications, such as driving a compressor or a pump, using DC motors is generally more advantageous then using AC motors. For example, compressor start-up requires a high torque at low speed start-up, which typically cannot be managed well by AC motors, especially AC motors having open loop controllers. Additionally, at lower compressor loads, the compressor may be more efficiently run at lower speeds. Typically, DC motors handle variable speed and high torque, low speed applications more effectively than do AC motors and control systems.
Therefore, in applications requiring high torque, low speed operation, a variable speed DC motor is generally used, particularly a brushless DC (BLDC) motor. However, one difficulty with using a BLDC motor is detecting a locked motor rotor, such as from motor bearing failure or debris blocking movement or damaging the phase coils. Another difficulty is detecting a rotor stopped from the loss of rotor phase lock, which the motor controller requires for proper electronic commutation of the phase coils in a BLDC motor.
While many applications use a rotor position sensor for maintaining rotor phase lock or for detecting a locked rotor, space and environmental considerations in sealed applications, such as pumps or compressors, limit the use of such a sensor. For example, a hermetically sealed compressor generally includes lubricant and refrigerant which, when heated by compressor operation and exposed to moisture, can form an acid that is corrosive to sensors. Also, if a motor sensor is used, additional electrical connections must be carried through the wall of the hermetically sealed chamber without compromising the hermetic sealing, adding further expense and an additional possible point of failure of the hermetic seal.
One common solution to detecting a locked or stopped rotor in electric motor applications is sensing a motor current drawn by the motor coils that is higher than the normal current of a running motor. For example, a locked rotor condition in a typical nonvariable electric motor may cause the motor current to exceed 80 amps, while normal high-speed motor operation only requires 10 to 15 amps. However, in the case of a BLDC motor, a locked or stopped rotor condition may draw less current than normal running current, for example, 30 amps for a stopped rotor, while normal high RPM operation may draw 40 amps. Thus, conventional over-current sensing detection does not provide a solution for detecting a locked or stopped rotor.
In compressor applications, in addition to bearing failure, debris, or lost rotor lock, another typical problem that may cause a BLDC motor to not run properly is an attempt to restart the motor shortly after compressor shutdown. After shutdown, the refrigerant head pressure of the compressor may provide more resistance than the available torque of the electric motor can overcome, thus causing a temporary locked rotor condition. Over time, head pressure eventually equalizes throughout the system, freeing the motor and compressor to again operate normally.
What is needed is a BLDC motor controller that provides locked and stopped rotor detection without the use of a rotor position sensor or over-current detection.